Laboratory measurements of artificial rain impinging on a water surface

In the wind-wave tank of the University of Hamburg laboratory measurements have been carried out in the presence of artificial rain and wind. By using a resistance type wire gauge the surface wave elevation, and by using a laser slope gauge the surface slope was measured. The radar backscattering from the water surface was measured with a coherent 9.8 GHz (X band) scatterometer. First results of this study show that the wave spectra are affected differently by the rain in different spectral domains. The wave spectra are enhanced in the frequency range of about 5 Hz to 100 Hz, while they are reduced at frequencies smaller than about 5 Hz. By comparing wave slope spectra measured in downwind and crosswind direction we found that the rain affects the orientation of the wave spectra: the transition region from an isotropic to an anisotropic wave spectrum increases with wind speed. By operating the scatterometer at different polarizations (VV, HH, and HV) we measured a strong increase of the relative backscattered radar power (σ_rel) in the presence of rain at wind speeds below approximately 8 m/s at all polarizations. At wind speeds above 8 m/s, only σ(rel) at cross-polarization increases in the presence of rain. The strongest increase, however, was measured at cross-polarization, which is in accordance with observations which we obtained from the analysis of multifrequency-SAR data